Alternating-current generator.



PATENTED MAY 9, 1905.

E. F. W- ALBXANDERSON. ALTERNATING CURRENT GENERATOR.

APPLICATION FILED SEPT. 14, 1904.

Inventor- Er'nstF W. A lexanderson UNITED STATES Patented May 9, 1905.

PATENT OFFICE.

ERNST F. WV. ALEXANDERSON, OF SOHENEOTIA'DY. NEIV YORK, ASSIGNOR TO GENERAL ELECTRIC COMPANY, A CORPORATION OF NEWV YORK.

ALTERNATlNG-CURRENT GENERATOR.

SPECIFICATION forming part of Letters Patent No. 789,404, dated May 9, 1905.

Application filed September 14, 1904. Serial No. 224,360.

To (all whm'n, it may 0072,0077.-

Be it known thatI, ERNST E. l/VJXLEXANDER- soN, a subject of the King of Sweden and Norway, residing at Schenectady, county of Sehenectady, State of New York, have invented certain new and useful Improvements in Alternating-Current Generators, of which the following is a specification.

My invention relates to self-exciting and I self-compounding alternating-current generators of the type in which the field-winding is excited, in whole or in part, by current taken from the armature and rectified by means of the commutator; and its object is to provide means for preventing sparking due to a shifting of the point of commutation with varying load.

In an application, Serial No. 221,322, filed by me August 19, 1904:, I have described a novel form of self-exciting and self-compounding generator in which the field-winding is supplied with current taken from the several phases of the armature through series and potential transformers, in which sparking at the 5 commutator is prevented by rendering the secondary circuits of the transformers substantially non-inductive and by so arranging the commutator that the circuit of the field-winding is never opened, but is simply transferred 3 from one phase to another at the instant when the two phases are at the same voltage. \Vith the arrangement described in my former application it is evident that if the phase of the exciting-voltage varies with respect to the point of commutation perfectly sparkless commutation will no longer be obtained, since the transfer from one phase to another will no longer occur at the instant when the two voltages are exactly equal. Some variation 4 of this kind normally occurs with the arrangement of compounding transformers described in my former application, since for non-inductive load the potential derived from the series transformers is at right angles to the potential derived from the potential transformers, the resultant of these two beingimpressed upon the. commutater. Neglecting armature reaction and self-induction,the phase of the potential derived from the potentialtransformer is fixed relative to the position of the field-poles, and consequently to the position of the commutator-segments. The resultant potential impressed upon the commutator, since it is the resultant of the potential from the potential-trapsformer and a series potential at right angles to it, will vary in phase relative to the potential from the potential-transformer with variation in the magnitude of the potential from a series transformer thatisgvith theload. In other words, the potential impressed upon the commutator tends to vary in phase relative to the position of the commutator-segments that is, to the point of commutation--and consequently a tendency to sparking exists. The same result is produced with the variation of the power factor of the load.

I have said that neglecting armature reaction the variation in phase of the impressed potential, as above described, would be considerable in amount. The armature reaction, however, tends to balance this variation, since it shifts the phase of the induced potential, and consequently of the potential derived from the potentlal-transformers, relatively to the position of the poles and of the commutatorsegments. This is evident from the fact that armature reaction produces cross-magnetization of the field, tending to strengthen the flux at one tip of the field-pole and weaken it at the other. The result is the same as though the pole itself were shifted in space, and consequently the induced potential is shifted in phase relative to the physical position of the field-pole and to the position of the commutator-segments. This shifting due to armature reaction in machines as ordinarily designed is not sul'licient to completely balance the shifting in phase of the electromotive force impressed upon the commutator relative to the potential upon the potential -trans former.

My present invention consists in so arranging a machine that a low-reluctance path is offered to the cross-magnetizing armatureflux, the cross-magnetization is increased, and

the effective shift of the field-flux relative to the physical position of the field-poles is consequently also increased.

More specifically considered, my invention consists in so arranging the field structure as to offer a low-reluctance path for the armature-fiux at points between the poles produced by the fiel dwinding.

Still morespecilieally considered, my invention consists in providing small auxiliary poles placed between the usual field-poles, this construction being adapted to secure the results above set forth in the manner which will be hereinafter fully explained.

My invention will best be understood by reference to the accompanying drawings, in which Figure 1 shows diagrammatically a selfexciting and self-compounding generator arranged in accordance with my invention. Figs. 2 and 3 are explanatory diagrams, and Fig. at shows the construction of a four-pole field-magnet with the auxiliary poles.

Referring first to Fig. 1, A represents an armature of an alternating-current generator, which for the sake of simplicity I have shown as a Gramme ring. It will be understood, however, that in practice the usual distributed drum-armature winding placed in slots or holes in a laminated armature-body would be employed. The terminals of one phase are indicated by the letters a a and of the other phase by the letters (0 {4 F represents the revolving field structure, which carries a novel form of two-part commutator C. On the commutator bear two pairs of brushes or sets of brushes 1) Z/ and 6 6 These brushes are displaced from each other by ninety electrical degreesthat is, by an amount equal to the angular displacement of the phases of the armature-winding. The commutator is provided with two conducting-segments c c, the length of each of which is approximately equal to the displacement of the brushes-- that is, to ninety electrical degrees. The pertions of the commutator between the segments are made of insulating material. A source of two-phase current, as will be hereinafter described, is connected to the two pairs of brushes.

It will be seen that as one pair of brushes leaves the two segments the other pair reaches the segments, so that the circuit of the fieldwinding which is connected to the two segments is never opened, but is simply transferred from one phase to the other. This transfer takes place every ninety degrees, and if the brushes are properly placed the resultant potential impressed upon the field-winding F will be as indicated in full lines in Fig. 2. In this figure, 1 represents the potential of one phase, which is connected to the brushes 5 b, and 2 represents the potential of the other phase, which is connected to the brushes transfer from one phase to the other may occur at the point of intersection of the two curves, as indicated in the figure. At this point of intersection the two phases are at precisely the same potential, and consequently if the transfer could be made instantaneously at this point absolutely no sparking could be produced. In practice of course it is necessary that both pairs of brushes should be in contact with the same segment for more than an instant, since some contact-surface is required between the brushes and segments to carry the field-current, so that one brush must pass partly or wholly on to a segment before the other brush entirely leaves. During this instant one phase is short-circuited on the other; butI reduce the duration of this shortcircuit to its minimum value by making the segments approximately equal in length to the brush displacement, so that one brush begins to leave the segment at the instant the other brush begins to engage it. WVith this arrangement the duration of the short circuit may be made so small that in practice no sparking is visible. Furthermore, owing to the exceedingly great distance between segments flashing over on the commutator is im possible.

It will be seen from the drawings that one set of brushes, 7/, is connected through the resistance R to the secondary of the potentialtransformer I The resistance It is inserted in the circuit for the purpose that has been heretofore explained 0., to render the excitingcircuit non-inductive. The presence of this resistance R enables the secondary circuit of the transformerP to be broken by the brushes 6 b with practically no sparking. In order to compound properly for varying loads and power factors, I have shown a series transformer T with its secondary connected across the terminals of resistance R. Connecting the series transformer in shunt to the resistance in this manner not only leaves the exciting-circuit non inductive, but also insures proper regulation of the current in the exciting-circuit relative to the current in the main circuit, as is fully disclosed in my former application.

It will be seen from Fig. 1 that the primary of potentialtransformer I is connected in one phase, while the primary of the series transformer T is connected in the second phase. For a non-inductive load, therefore, when the current in each phase is in phase with its potential the secondary voltages of the two transformers will be ninety degrees out of phase. This is indicated in Fig. 8, in which the line 1) indicates the magnitude and phase of the secondary voltage obtained from the potential-transformer. The line 6' represents the magnitude and phase of the potential obtained from the secondary of the series transformer for a non-inductive load of a given amount. The vector sum of these two lines, which is represented by 6, represents the value of the potential impressed upon the brushes 6 6'. Now assume that the load remains constant in amount, but varies in phase so as to lag forty-five degrees. The secondary voltage of the series transformer will be represented by the line 6 and the resultant electromotive force impressed upon the brushes will be represented by the line a. It will be seen that this line is greater than the line 6. Thus the electromotive force impressed upon the brushes is increased as the power factor decreases. With a power factor of zero the secondary of the series transformer will be represented by the line 6 and the potential impressed upon the brushes would be the algebraic sum of the lines p and t. From this it will be seen that by properly proportioning the potential and series transformers the voltage impressed upon the field-winding may be properly varied to compensate for varying power factors. Similarly, it is evident that if the line 6 or t in Fig. 3 be increased or decreased the length of the line 6 or a will be also increased or decreased. Thus this arrangement automatically compounds for varying load, as well as for varying power factor. The potential and series transformers P and T form a similar arrangement for the second pair of brushes 6 so that a twophase excitation is impressed upon the two sets of brushes, so as to give an impressed electromotive force on the field, as shown in Fig. 2. It should be understood that Fig. 2 represents the potential impressed upon the field and not the current. Since the self-induction of the field-circuit is very great, there is a tendency to smooth out even the small fluctuations in impressed voltage shown in Fig. 2, so that the current-curve would be represented by a nearly-straight line.

So far the question of the shifting of the point of intersection of two waves of different phases has not been considered. By referring to Fig. 3, however, it will be seen that a change either in the magnitude or in the phase of the current in the armature-circuit that is, a variation in the length or position of the line twill produce a change not only in the length but also in the position of the line 6 relative to the linep. The line p,which represents in phase as well as in magnitude the voltage of the armature-circuit, is fixed with reference to the position of the field structure in space, if the effect of armature reaction and self-induction be for the moment neglected, which means that it is fixed with respect to the position in space of the commutator-segments c 0. In other words, the variations in the magnitude and phase of the armature current shift the potentials impressed upon the commutator-brushes with respect to the point of commutation. Consequentl y commutation no longer occurs after such a change at the point of intersection of the two current-waves, and this change is productive of a tendency to spark. As has been said heretofore, the armature reaction tends to counterbalance this shifting of the impressed potential on the commutator relative to the point of commutation by shifting the potential of the effective flux with respect to the physical position of the field-poles. This is indicated diagrammatically in Fig. 1. Vith the polarity of the field as indicated by the letters N and S, with the direction of rotation of the field structure as indicated by the arrow, and assuming the armature-current to be in phase with the induced voltage, the current induced in the portion of the armaturewinding opposite pole N will produce a crossflux in the pole-piece, as indicated by the arrow-heads. It will be seen that this flux opposes the field-flux at the leading pole-tip and strengthens it at the trailing pole-tip, or, in other words, tends to crowd the field-flux into the trailing portion of the pole. The result of this action, as regards the induced potential. is exactly the same as though the pole were actually shifted backward a certain amount in space, so that the induced potential no longer corresponds in phase with the physical position of the pole-piece. In other words, the potential derived from the potential-transformer, as indicated by the line a in Fig. 3, is shifted with respect to the point of commutation, and this shift tends to maintain the impressed electromotive force (2 of Fig. 3 at a constant phase position relative to the point of commutation, and just as the angle between the lines a and p increases with increasing load so also the cross-magnetization of the pole increases with increasing load. Furthermore, just as the angle between the lines 7) and 6 decreases with decreasing power factor, as indicated in Fig. 3, so, too, the cross-magnetization decreases with decreasing power factor, the wattless component of the armature reaction appearing as a demagnetizing, not a cross magnetizing, magnetomotive force,as is well understood in the art. Thus for varying loads and varying power factors the armature reaction tends to compensate for the shift of the impressed electromotive force on the commutator relative to the point of commutation. With machines as ordinarily designed, however, the armature reaction is not of sulficient amount to wholly compensate for this shift. If, however, the field-magnet is arranged to offer a low-reluctance path for the cross-magnetizing fluxas, for instance, if auxiliary field-poles a and s are placed on the field structure, as shown in Fig. 1the crossflux of the armature will be increased, the direction of the flux being as indicated by the arrows and the polarity produced in these auxiliary poles being as indicated by the let ters n and s. This auxiliary pole at, trailing behind the main pole N, has the efiect of still further lagging the position of the effective flux relative to the physical position of the pole N. Furthermore, the strength of the pole n, and consequently the lagging effect produced by it, varies with variation in armature-current strength. Evidently if the reluctance of the auxiliary poles is properly proportioned the shifting tendency of the electromotive force impressed upon the commutator relative to the point of commutation may be wholly compensated for.

Fig. 4 shows an auxiliary pole placed between two poles N and S of the four-pole machine. (0 represents a coil of one phase of the armature-winding, which for the sake of simplicity is represented as a concentrated winding. If the polarity of the field-poles is as indicated by the letters Nand S and if the direction of rotation of the field-magnet is as indicated by the arrow, the magnetomotive force due to the coil a will be as indicated by the arrows crossing the air-gap. 'It will be seen first that these magnetomotive forces weaken the flux at the leading pole-tips of both poles and strengthen the flux at the trailing pole-tips. It will further be seen that the auxiliary pole s offers an additional path for the cross magnetizing flux, so as to strengthen the armature reaction, and that the direction of the armature-flux through the pole s is such as to give it a polarity as indicated by the letter by which it is designatedthat is, it forms a trailing south pole varying in strength with the armature-current and having the effect of increasing the lag of the effective field-flux relative to the physical position of the field-pole. l Vith proper proportioning of the reluctance of the auxiliary poles the point of commutation may be maintained absolutely constant relative to the elec tromotive force impressed upon the commutator and sparkless commutation thereby obtained for all loads and power factors.

Obviously the employment of auxiliary projecting poles is not essential for the purposes of my invention. Any other disposition of the magnetic material of the field structure which offers a low-reluctance path for the cross-magnetizing flux of the armature, particularl y at points between the poles produced by the field-winding, is sufiicient for the purposes of my invention. Accordingly I do not desire to limit myself to the particular construction and arrangement shown, but aim in the appended claims to cover all modifications which are within the scope of my invention.

I/Vhat I claim as new, and desire to secure by Letters Patent of the United States, is*

1. In an alternating-current generator, a field structure adapted to ofler a low reluctance to the cross-magnetizing flux due to the armature-current, a field'winding provided with a commutator, and means for supplying to said commutator a current derived from the armature.

2. In an alternating-current generator, at field-winding provided with a commutator, and means for impressing thereon an electromotive force derived from the armature and varying in phase relatively to the electromotive force induced in the armature as the load varies, the field structure being adapted to offer a low reluctance to the crossmagnetizing flux due to the armature-current, whereby the phase of the effective field-liux is shifted relative to the physical position of the commutator to balance the variation of the electromotive force impressed on the commutator relative to the electromotive force induced in the armature.

3. In an alternating-current generator, a field-magnet with projecting poles, a lieldwinding on said poles provided with a commutator, auxiliary poles on said magnet between the main poles, and means for supplying to said commutator an alternating current derived fromthe armature.

4. In an alternating-current generator, a field -magnet with projecting poles, at lieldwinding on said poles provided with a commutator, means for supplying to said commutator an electromotive force derived from the armature and varying in phase relative to the induced electromotive force of the armature as the load varies, and auxiliary poles on the field-magnet between the main poles adapted to offer a path for the cross-magnetizing flux due to the armature-current, whereby the phase of the effective lield flux is shifted relative to the physical position of the commutator to balance the variation of the electromotive force impressed on the commutator relative to the induced electromotive force of the armature.

5. In an alternating-current generator, a field structure adapted to offer a low reluctance to the cross-magnetizing flux due to the armature-current, a field provided with a commutator having two segments per pair of poles, brushes bearing on said commutator, and means for impressing on said brushes polyphase voltages derived from the armature, the length of each commutator-segment being approximately equal to the angular displacement between the phases of said voltages.

6. In an alternating-current generator, a field structure provided with projecting poles, a field-winding on said poles provided with a commutator having two segments per pair of poles, brushes bearing on said commutator, means for impressing on said brushes polyphase voltages derived. from the armature, the length of each comm utator-segment being approximately equal to the angular displacement between said phases, andauxiliary poles IIO between the main poles adapted to offer a path for the cross-magnetizing flux due to the armature-current, whereby the phase of' the effective field-flux is shifted relative to the physical position of' the commutator as the load varies.

7. In an alternating-current generator, a polyphase-armature winding, a field-magnet adapted to offer a low reluctance to the crossmagnetizing flux due to the armature-current, a field-winding provided with a commutator, brushes bearing on said commutator, and means for impressing on said brushes polyphase voltages each formed of two components proportional to the voltage and current respectively of different phases of the armature, and said components of each voltage being combined at right angles to each other for non-inductive load.

'8. In an alternating-current generator, a polyphase armature, a field-magnet having projecting poles, a field-winding on said poles provided with a commutator, brushes bearing on said commutator, means for impressing on said brushes polyphase voltages each formed of two components proportional to the voltage and current respectively of different phases of the armature and said components of' each voltage being combined at right angles to each other for non-inductive load, and auxiliary poles on said field-magnet adapted to offer a path for the cross-magnetizing com-- ponent of the armature-flux, whereby the phase of' the effective field-flux is shifted relative to the physical position of the commutator as the load varies.

9 In an alternating-current generator, at

field-magnet adapted to offer a low reluctance to the cross-magnetizlngflux due to the armature-current, a field-winding provided with a 4 phase to another at the instant when both 45 phases are at the same voltage.

10. In an alternating-current generator, a field-magnet having projecting poles, a fieldwinding on said poles provided with a commutator, means for impressing on said co m- 5 mutator polyphase voltages derived from the armature, said commutator being arranged to transfer the field-winding from one phase to another at the instant when both phases are at the same voltage, and auxiliary poles on the 5 5 field magnet intermediate the main poles adapted to offer a path for the cross-magnetizing flux due to the armature-current.

11. In an alternating-current generator, a

field structure adapted to offer a low-reluc- 60 tance path for the fluX due to the armaturecurrent at a point between the poles produced by the field-winding, a commutator carried by the field structure and connected to the field-Winding, and means for supplying to 5 said commutator a current derived from the armature.

In witness whereof I have hereunto set my hand this 10th day of September, 1904.

ERNST F. W. ALEXANDERSON.

l/Vitnesses:

BENJAMIN B. HULL, HELEN ORFORD. 

